A gas compressor may be driven by a ground base gas turbine having a single or twin shaft arrangement. In a single shaft arrangement the gas turbine include at least an inlet compressor and a power turbine mounted on the same shaft, on which also the gas compressor is mounted.
In a twin shaft arrangement the gas turbine comprises a so called ‘gas generator’ including a first shaft and a power turbine mounted on a second shaft. The gas compressor is also mounted on the second shaft. A gas generator typically comprises an inlet compressor, a compressor turbine, both mounted on the first shaft, and a combustor between the inlet compressor and the compressor turbine. A gas generator can also consist of more than one pair of compressor and compressor turbine. The gas generator is used for delivering gas at certain conditions of pressure and temperature to the power turbine, by means of a connection between the outlet of the compressor turbine and the inlet of the power turbine.
Gas compressors driven by ground based gas turbines, either with single or twin shaft arrangement, typically use so called ‘dry gas seals’ on their rotors to prevent leakage of the gas being pumped. Typically a dry gas seal comprises a stationary ring and a rotary ring having a planar annular surface which faces the stationary ring and which is provided with grooves. A gas is normally present in the very fine clearances between the stationary ring and the surface of the rotary ring provided with grooves. In operation, when the rotary ring rotates with respect to the stationary ring, the viscosity of the gas flowing in the grooves maintains a thin cushion between the rings, thus preventing wear. The main drawback of dry gas seals is that, below a minimum speed, for example in a 5-15MW compressor system such minimum speed being typically below 400 rpm, a suitable gas cushion does not form and contact occurs. Therefore, it should be desirable to prevent the compressor spinning below the minimum speed. Spinning below the minimum speed required by the seals to work properly is normally referred to as ‘slow roll’.
It is nevertheless also necessary that gas turbines are spun by the starter system, post shutdown, at a low speed to cool the engine and to prevent distortion due to temperature gradients in the rotors, which can cause vibration problems and interferences at the subsequent re-start. Failure to spin the rotor soon after shutdown, the time is dependent on the size of the gas turbine, typically within minutes for a small gas turbine e g 15 minutes, can cause appreciable delays to restarting the engine (such delay being normally referred to as ‘lockout’) or can risk damage to compressor blades and seals.
For single shaft gas turbines the gas turbine rotor and the gas compressor is typically mechanically connected, e.g. via a clutch and so can be made to rotate together driven by the starter system, while on twin shaft gas turbines, the starter system directly drives the inlet compressor of the gas generator. The spinning of the gas generator can provide sufficient flow to cause the power turbine and hence the gas compressor to slow roll under some or all circumstances, depending on the driven unit and installation details, as well as variations in weather conditions. Therefore, avoiding slow roll on twin shaft gas turbines can be particularly demanding.